Why do people say this? Not to pick on you, but I have heard a lot of people say this, and I just don't get it.
The sound from fingers can be every bit as aggressive as the sound from a pick. Geddy Lee plays a lot of stuff that has an aggressive attack. The only place I can see a pick having an advantage is playing rapid-fire 1/16 or 1/32 notes, but even there the advantage is not significant.
This clip was recorded at a jam. I played the bass using fingers. I really would like for someone to point out how this sound and attack is any less aggressive than what can be achieved with a pick -- and I'd like to hear clips demonstrating how pick playing is more aggressive in its attack than this.
IMO, if people can't get an aggressive sound from their bass using fingers, it's a failure of their application of technique, not a failure of the technique itself.